Endodontics, also known loosely as root canal therapy (RC) is a major part of dental treatment especially where saving an infected tooth is concerned. Endodontic instruments used to negotiate the canals inside the roots of teeth consist primarily of files and reamers. These instruments may be machine-driven, in which case a grasping end or shank is firmly grasped in a machine collet or chuck, such as in a rotary drill, a reciprocating filing machine or in an ultrasonic vibrating machine. Because of the sensitive nature of the root canal procedure and the small sizes of the cutting instruments, hand-held files and reamers are popular. Typically, hand-held files and reamers are provided with miniature or finger-size handles as a grasping end by which the dentist grasps the non-working end of the instrument for manipulation within the tooth nerve canal.
Files are elongated, round endodontic instruments designed for removing dentin from the inside of a tooth primarily with a vertical movement in the axial or elongated direction. Reamers are similarly elongated, rounded endodontic cutting instruments; however, they are designed primarily for enlarging or smoothing a hole in a tooth by the removal of dentin from the inside of the tooth primarily through rotation. Because of the difference in operation (i.e., an up-and-down movement, compared with rotational movement), files typically have a greater number of cutting edges and a higher twist angle and reamers have fewer cutting edges and a lower twist angle. Under the ISO standards, a file typically has four cutting edges formed as by twisting a small shaft having a square cross-section such that each of the four corner forms a cutting edge. The flat faces of the square form clearance flutes. A reamer is typically formed by twisting a triangular cross-sectional-shaped shaft such that it has three edges. The flat faces of the triangle form clearance flutes. The high twist angle of the files provides the cutting edge with a significant horizontal component relative to the up-and-down movement. The low angle of twist on a reamer provides the cutting edge with a significant vertical component for cutting by rotational movement. Many manufacturers have presented differently shaped cutting edges and flutes in order to provide different cutting characteristics.
U.S. Pat. No. 5,017,138 has, in its Background of the Invention, a summary of how endodontic instruments are standardized through the International Standard Organization (ISO). Basically, the shape, length, size and handle color of endodontic instruments are defined by ISO's Standard 3630. FIG. 1 shows a prior art endodontic cutting instruments basically according to the current ISO standards.
Pursuant to Standard 3630, the diameter of the working tip of an endodontic file (or reamer) corresponds to the number that is used to identify the file. For instance, an ISO standard size 10 means that the working tip diameter, so called "D.sub.1 " (as opposed to "D.sub.2 " which is the shank, diameter further up the file), measures 0.10 mm. The next larger standard size file would be No. 15, indicating a working tip diameter of 0.15 mm at D.sub.1. The next larger standard size is 20, then 25, then 30, then 35 and so on, indicating 0.05 mm incremental size changes from one standard size to the next larger standard size until size 60 is reached. After size 60, then diameters at D.sub.1 increase by 0.1 mm for each standard size increment so that the next standard size file greater than size 60 is size 70 with a diameter D.sub.1 of 0.75 mm. Then, 80, 90, 100 and so on every 0.10 mm until size 150. However, apparently the largest size commonly available on the market is 140.
Other than the standardization of the working tip diameters and the standardization of incremental size difference between each diameter, the other noteworthy feature about the ISO standard is that it establishes an accepted international standard color scheme for the endodontic file and reamer handles. These colors are fixed and then correspond to the standard sizes of the tip diameters for the standardly tapered endodontic cutting instruments. Six basic colors are used, and accordingly to the ISO scheme, are repeated. The colors are: white, yellow, red, blue, green and black. Files outside of the standard range are sometimes available from various manufacturers, but there has been no accepted standard color scheme for differentiating the non-standard sizes from the standardized sizes or for differentiating the taper which is standard. Some manufacturers have developed their own unique colors, and some have merely printed different size measurements on the handles. Confusion has not been avoided.
Dentists and dental assistants can still depend on the fact that an ISO size 15 file will be white in color. This is also the file that most root canals are started with. When the dentist begins to negotiate the length of the root canal completely and reach the root tip with a size 15 file or reamer without blockage or binding, the next larger file, size 20 (which is yellow in color) is grasped or handed to the dentist by an assistant for removing additional material until the tip is reached. The process is repeated, step-wise, with the next larger size 25 (which is red). Then, the size 30 (which is blue), the size 35 (which is green) and followed by size 40 (which is black). The size differences and color-coding are set forth in Table I, below.
TABLE I ISO STANDARD COLOR-CODE SYSTEM Diameter of Instrument Size D.sub.1 (mm) D.sub.2 (mm) Color 10 0.10 0.42 Purple 15 0.15 0.47 White 20 0.20 0.52 Yellow 25 0.25 0.57 Red 30 0.30 0.62 Blue 35 0.35 0.67 Green 40 0.40 0.72 Black 45 0.45 0.77 White 50 0.50 0.82 Yellow 55 0.55 0.87 Red 60 0.60 0.92 Blue 70 0.70 1.02 Green 80 0.80 1.12 Black 90 0.90 1.22 White 100 1.00 1.32 Yellow 110 1.10 1.42 Red 120 1.20 1.52 Blue 130 1.30 1.62 Green 140 1.40 1.72 Black 150 1.50 1.82 White
The six colors are repeated again after size 40 (which is black) with ISO standard size 45 being white again. The standard size 50 is yellow, size 55 is red and size 60 is blue. As stated above, from standard size 15 to standard size 60, the standard incremental increase in working diameter is 0.05 mm. From size 60 to size 150, the standard incremental working diameter increase is 0.10 mm. The standard color scheme repeats sequentially for each next larger standard size. Since there is no standard size 65, size 70 is the next color in the sequence (i e., green), size 80 is black. Size 90 initiates the repetition of the sequence at white; then size 100 is yellow, size 110 is red, size 120 is blue, size 130 is green, through size 140 which is black. Size 150 is white, but is seldom used and is seldom commercially available. From size 15 to size 140, there are eighteen different standard endodontic instrument sizes for files and reamers under the ISO system. The color scheme of white-yellow-red-blue-green-black repeats three times.
If the dentist encounters a problem negotiating the root canal with a size 15 instrument, there are now three smaller ISO-recognized standard sizes--10, 8 and 6--with corresponding working tip diameters of 0.1 mm, 0.08 mm and the very fine 0.06 mm. They bear special ISO colors of purple, silver and pink, respectively.
This color scheme is taught to dental students from the beginning of their studies in endodontics. By the time the student graduates from dental school, he or she invariably has become very familiar with the scheme for standard sizes. If a white file is encountered, he or she immediately recognizes it as either a size 15, 45 or 90. A very large ISO file would be obvious. The size difference from one standard whole size to the next (i.e., 0.05 mm or 0.10 mm increase) can be difficult to distinguish by mere visual observation without a color code. With six standard whole sizes between each repeated color, there is a sufficient size variation for most dentists and trained personnel to distinguish between each of the files of the same color even if the complete set of files becomes mixed as on a tray out of sequence.
Manufacturers of endodontic files typically color code and number their files and reamers on the handles. However, during an endodontic procedure, observation of small printed numbers is frequently obscured by materials or the dentist's fingers. It is noted that the file handles are typically only about 1 cm in length and about 0.5 cm in diameter. It is also noteworthy that the standardization of incremental increases from one whole size to the next for endodontic instruments by 0.05 mm for sizes 15 to 60 and by 0.10 mm for sizes 60 to 140 is a collective effort on the part of manufacturers and practitioners in the art. As with most well-meaning efforts, some controversy and problems have arisen. The 0.05 mm incremental increase, in some cases, has been too much. That is, sometimes where a canal can comfortably be negotiated by one standard size, it will not allow the negotiable by the next larger size which is at least 0.05 mm larger for each standard incremental size.
Recognition of this problem is not new. In the seminal book, Endodontic Practice(11th Ed.), by Grossman, et al., the authors discuss a hypothetical, yet commonly encountered, situation. In the scenario offered by the textbook, a size 10 file will go the distance of the canal but a size 15 file will not. The authors suggest that, "[because the next size file needed is not available commercially, the operator can create one by cutting off part of the instrument tip of the size 10 file. All standardized instruments taper 0.02 mm in diameter per 1.0 mm of blade length. Cutting off 1.0 mm of tip of a size 10 file converts it to a size 12 file." (p. 208) Of course, the resulting working tip, which was originally at a 75.degree. angle per ISO standards, may cut unevenly unless great skill and care is exercised to recreate a symmetrically-angled cutting tip.
More than one Endodontic textbook as well as numerous dental seminars have taught the above-mentioned method of creating "intermediate" files to the ISO system. The problem of not being able to go the distance with the next larger file has also created the problem of broken files: As the next larger file is just a little short of the length of the canal, it is not uncommon for the operator to push a little harder. The file then binds and in an attempt to pull the file out, it breaks. This has led to the oft-quoted rule in endodontics of "No more than one quarter turn" in using files and reamers.
As a result of the foregoing, many dental articles have been written about how to retrieve broken files as well as implements for sale to dentist to do same. This is also the backdrop of the U.S. Pat. No. 5,017,138.
The '138 patent criticizes the ISO's system of fixed 0.05 mm incremental increase as providing a non-uniform percentage increase in size over the 0.1 mm to 1.4 mm range. The proposed solution is to offer uniform percentage increases in files rather than a fixed amount of increase. Under the proposed '138 uniform percentage increase scheme, each subsequent file would increase the same percentage in size as oppose to an irregular percentage under the ISO system.
The 0.05 mm incremental increase under the ISO mathematically does have an irregular percentage increase in file size. For instance: from size 10 to size 15, that is from 0.10 to 0.15, a 0.05 mm increase equates a 50% increase. From 15 to size 20, that is from 0.15 to 0.20, a 0.05 increase equals a 33% increase. This irregular, albeit decreasing percentage, was pointed out in the '138 patent. However, where the '138 patent fails is that even in a thirteen-instrument set (the widest selection in the examples given), for a uniform increase of 29%, after the tip size reaches 0.167 mm (instrument 5), the size increase up to the next size of 0.216 mm (instrument 6) is 0.049 mm [column 4, line 6 of the '138 patent]. We are essentially back to the ISO increase of 0.5 mm.
Above Instrument 6, the 29% increase translates to a whopping 0.063 mm between size (instruments 6 and 7): 0.216 mm and 0.297 mm [column 4, line 7]. Then a gargantuan leap of 0.087 mm. between sizes 0.297 mm and 0.366 mm (instruments 7 and 8). By the time instrument 13 is reached with a D.sub.1 of 1.293 mm. (similar to an ISO size 130 file), there is a giant 0.293 mm increase over instrument 12 [column 4, line 54]. This is three times the 0.10 mm increase under the ISO system!
Another important disadvantage of the '138 system is that it involves a totally different color scheme. At this writing, one manufacturer has offered for sale a series of 13 files based on the '138 patent. The manufacturer used its own color code in which four colors are repeated twice plus five other colors for the set of thirteen instruments. These nine new colors include several hues of brown and green. There are also two different reds, plus a pink.
It is reasonable to think that the manufacturer of the '138 set of files is trying hard to prevent any overlap of colors with the ISO colors. Imagine the dentist (or, more likely, the chairside assistant) haplessly trying to find the next larger size with file colors closely resembling one another. However, in creating new colors, the manufacturer also has unwittingly forced the user of these new files to memorize a new set of colors. Habits are hard to form and even harder to forget.
The problem with the 0.05 mm. increase under the ISO system is also drawing the attention of another manufacturer. Recently, a new set of endodontic files has been introduced which are the "half-sizes". No longer do dentists have to bring along a fingernail file in their endodontic boxes to clip file tips in order to make custom intermediate sizes. New non-standard half-sizes of 22.5, 27.5, 32.5, 42.5, and 47.5 corresponding to 0.225 mm, 0.275 mm, 0.325 mm, 0.425 mm and 0.475 mm tip diameters have been introduced. However, this only addresses part of the problem.
The problem relating to the identification of half-sizes has not been adequately addressed. These non-standard size files are in-between existing standard sizes or intermediate sizes and need to be identified properly in order that the user can quickly select the appropriate one during the root canal procedure. As with the '138 file set, a totally different color scheme will be confusing at best and will likely never be adopted or relearned. The mere printing of sizes on the small handles is inadequate in a realistic situation. More than one intermediate size in critical working ranges has not been previously proposed.
The soft pulp, which extends substantially along the center of each root of the tooth, is surrounded by a relatively harder tissue known as dentin. In root canal therapy, the pulp is to be completely removed along with adjacent portions of dentin which may harbor decay or bacterial infection. The removal of the pulp and adjacent dentin forms a root canal cavity, which cavity is then preferably cleaned and sealed and then filled with a filler material so that no open spaces, pockets or voids remain. Any spaces, pockets or voids could permit an environment for infection to reappear. It is therefore important that the root canal cavity be formed smoothly tapered from the crown of the tooth to the tip of the root. In modern dentistry, the preferred filler material is gutta percha. It is also preferable that the apical opening at the very tip of the canal be sufficiently large to remove all decay and yet sufficiently small so that gutta percha can be filled and compacted tightly into the root canal cavity.
In a typical root canal procedure, a small diameter endodontic tool is first used to make a very small canal which reaches the very tip of the root. Subsequently, progressively larger endodontic files are used to progressively enlarge the canal cavity and to remove all infected material. The depth of insertion of each instrument is carefully monitored. It is important that the cavity at the apical foramen is not made larger than necessary. It is also important that the alveolar bone is not penetrated. Traditionally, a procedure known as "step back" allows the length of the cavity to be sufficiently large without opening the apical orifice too much. When the dentist is certain that all of the pulp and any potentially infected or decayed dentin is removed, gutta percha will be packed into the root canal cavity. The gutta percha can be easily inserted and packed against the small opening acting as a dam so that it fills the entire cavity without the formation of spaces, pockets or voids. The step-back procedure leaves a sufficiently small apical orifice yet rapidly flares or tapers outwardly into the dentin to form a larger cavity diameter, which easily accepts the gutta percha material. Advantageously, this flare or necking at the apex both follows the typical anatomical shape of the softer pulp and adjacent dentin along the root of the tooth, and it also acts as an obstruction or a dam against which the gutta percha can be compressed. Sufficient pressure can be exerted to fill the cavity completely.
In a step-back procedure, therefore, when the desired size of the apical orifice is achieved, then each successively larger file or reamer is inserted and used to a shallower depth than the preceding smaller file or reamer. For example, each next larger size is inserted to a depth which is one to one-and-one-half millimeters less than the depth of the preceding file. Each standard file has a taper of 0.02 nm/mm. Thus, the cavity formed also has a similarly slight taper, less than about 20.degree.. A steeper taper angle of between about 0.05 nmu/mm and 0.1 mm/mm (between about 3.degree. and 6.degree.) is achieved at the tip of the cavity using a standard step-back procedure.
With skill and practice, the dentist or endodontic specialist could use multiple drill sizes to perform the function of stepping back leaving only small ridges from one size to the next. However, the process of moving from one file to the next and carefully measuring and maintaining the different depths to which each subsequently larger file may be inserted is a time consuming and tedious process. In the event that significant ridges are inadvertently left between one step and the next, the step back, although approximated, could present the opportunity for infection harboring pockets at the ridge comers. Progressively smooth transition from the large diameter root canal cavity to the apical opening is the objective. Thus, a steeper angle than the normal or standard cutting edge tapered angle is desirable at the apical opening. Standard text book teaching for this procedure indicates a step back of three to five steps. Each step is approximately one-half to one millimeter in depth for each 0.05 millimeter increase in tip diameter for the endodontic tool. This results in a steep constriction in the root canal cavity extending one-and-one-half millimeters to about five millimeters from the apical opening into the root. The taper angle at that distal portion of the cavity will be approximately 0.05 mm/mm to 0.10 mm/mm (about 3.degree. to 6.degree.), which taper angle is substantially steeper than the approximate 0.02 mm/mm (about 1.degree.) taper, which is the ISO standard for the cutting edge taper angle for endodontic tools. This is in contrast to the standard endodontic cutting instrument tip bevel angle of 60.degree. to 75.degree.. This standard angle is the same for the terminal tip end of both files and reamers.
The present invention provides a working portion of the files which has a portion of the cutting edge which is at an angle which is steeper than the standard ISO angle. The unique instrument structure provides the desired step-back angle extending a sufficient distance back from the tip. The normal physiology of the dentin to be removed is closely approximated with an initially sharp angle and then a subsequently standard shallow tapered angle.
One attempt to facilitate step back is disclosed in U.S. Pat. No. 1,443,193, in which the tip of the file rather than the working portion of the file is tapered into a "cutting means" at an angle which is less than the ISO standard 75.degree. angle. The tip cutting means does not cut as well as the normal cutting edge. This does not provide an adequate solution because the resulting step back is both at a very steep angle usually about 450 and it has a depth which is less than the diameter of the cutting tool, usually less than about 0.5 mm. Thus, the result is a sharp bevel rather than a smoothly tapered step back extending from a depth of one-and-one-half millimeters to five millimeters.
Recently, some manufacturers have provided endodontic tools for power rotation rather than for hand manipulation which have non-standard cutting edge angles. For example, a steeper angle is formed along the entire cutting edge. Thus, these tools result in a steeper taper along the entire root canal cavity. A wide coronal opening results and the volume of dentin removed and of gutta percha filled is larger and therefore more time consuming. A device and method for forming a step back having a standard size and shape, but using a single step procedure, is needed.